A refrigerant flow rate controller utilizing a thermoelectric expansion valve, which is a type of electronic motor-driven valve is disclosed in Japanese Patent Publication (sho) 58-47628 (IPC, F25B41/06) and published in "reitoh" (refrigeration) PP. 60-64, Vol. 56, No. 641 (March, Showa 56). This refrigerant flow rate controller has a first temperature sensor positioned at or near the refrigerant inlet of the evaporator and a second temperature sensor at the refrigerant outlet of the evaporator. The electric signals from the sensors are compared with control of the refrigerant flow rate based on the difference between them so as to maintain the opening of the expansion valve and consequently to keep the superheating in the evaporator approximately constant.
In addition to the above, i.e. the electronic motor-driven valve control for the superheating, temperature control of the space to be cooled (hereinafter referred to refrigeration space) has been introduced in U.S. Pat. No. 4,745,767. This refrigerant flow rate controller is described below with reference to FIGS. 1-3.
FIG. 1 shows a schematic construction of the overall refrigerating system for use for example with a low-temperature show case installed in a supermarket. FIG. 2 is a block diagram of the control circuit of the refrigerant flow rate controller. FIG. 3 illustrates the variation in temperature of the refrigeration chamber under control of the controller, as a function of the operation of the electronic motor-driven valve.
As FIG. 1 shows, a compressor 1, a condenser 2, an electronic motor-driven valve 3, and an evaporator 4 are connected by tubes to form a closed loop of refrigerant circuit 5. The electronic motor-driven valve 3 is a pulse-driven electronic expansion valve. A fan 7 installed at a lower position of the cold air passage 6 of a low-temperature show case I takes in air from an air intake port 8a, which air is cooled by the evaporator 4 and discharged from an air discharge port 8b, to form an air curtain A to cut the influence of ambient air on the refrigeration space 9.
The electronic motor-driven valve 3 is controlled by a control signal a received from a controller 10 e.g. a micro-computer, such that opening of the value 3 permits regulated flow of decompressed refrigerant from the condenser 2 to the evaporator 4. Detector signals b.sub.1, b.sub.2, b.sub.3, and b.sub.4 control the electronic motor-driven valve 3. Detection signals b.sub.1, and b.sub.2 are obtained by forming electric signals from the temperatures detected by the inlet evaporator temperature sensor 11 at or near the evaporator 4 inlet and the temperature detected by the evaporator outlet temperature sensor 12, respectively. The detection signals b.sub.3 and b.sub.4 are obtained by forming electric signals from the temperature detected by means of the returned air temperature sensor 13 measuring the temperature of the air returning to the intake port 8a and a discharge air temperature sensor 14 measuring the temperature of the air discharge, respectively. When input in the controller 10, these signals b.sub.1, b.sub.2, b.sub.3, and b.sub.4 are processed therein and transmitted in the form of control signal a to electronic motor-driven valve 3. More particularly, the detection signals b.sub.1, and b.sub.2 from the evaporator temperature sensor 11 and evaporator outlet temperature sensor 12 concerns control of the superheating, while the detection signals b.sub.3 and b.sub.4 from the returned air temperature sensor 13 and discharging air temperature sensor 14 concerns temperature control of the air in the refrigeration space.
Specific control operations carried out with such detection signals b.sub.1, b.sub.2, b.sub.3, and b.sub.4 are as follows.
A refrigerant flow rate control unit S having structure shown in FIG. 2 includes a first comparator 15 for comparing a feed back signal with a reference value representative of required superheating, an inner algorithm section 16 for regulating internal relationships, a valve driver 17 for driving the valve, an evaporator temperature calculator 18 for calculating the temperature of the evaporator 4, a refrigeration space temperature calculator 19 for caluclating the measured temperature of the refrigeration space 9, a second comparator 20 for comparing the temperature of the refrigeration space 9 with its reference temperature, and a valve full-close signal generator 21 for fully closing the valve.
In the evaporator temperature calculator 18, the detection signal b.sub.2 from the evaporator outlet temperature sensor 12 and the detection signal b.sub.1 from the evaporator temperature sensor 11 are processed to give measured superheating (SH) which is in turn compared with the preset superheating (SHS) in the first comparator 15. The deviation of the former from the latter is input in the form of a deviation signal DV into the inner algorithm section 16 and corrected therein. The valve driver 17 receives this corrected signal as a regulating signal (HSS) and continually outputs valve opening regulating signal (BKC) to the electronic motor-driven valve 3 based on the deviation. The valve opening regulating signal (BKC) applied to the valve 3, which are pulse mode signals free of external perturbations (DT), result in appropriate mechanical regulation of its opening i.e. the cross sectional area of the valve, to thereby regulate of the refrigerant flow rate (GA) such that the superheating will remain within the preset value, say 5.degree. C. Such controlled regulation of the electronic motor-driven valve 3 is performed in steps during the pull-down operation period t.sub.a subsequent to defrosting operation and period t.sub.b of refrigeration in the thermocycle mode (which will be described in detail later), as shown in FIG. 3. Consequently, the measured temperature T.sub.M for the refrigeration space may reach the preset temperature T.sub.S. It should be born in mind that in each of the refrigerating periods t.sub.a and t.sub.b, opening of the electronic motor-driven valve 3 is regulated in steps. That is, although the open/close condition of the valve 3 is indicated as by ON and OFF, the actual opening changes in steps in accordance with the deviation of the measured superheating (SH) from the preset superheating (SHS).
On the other hand, the refrigeration space temperature calculator 19 calculates the temperature T.sub.M of the refrigeration space 9 from the detected signal b.sub.4 obtained from the discharging air temperature sensor 14 and the detection signal b.sub.3 obtained from the returned air temperature sensor 13. The measured refrigeration space temperature T.sub.M is compared in the second comparator 20 with the preset temperature T.sub.S. When T.sub.M .ltoreq.T.sub.S, a valve full-close signal (BP) is emitted from the valve full-close signal generator 21 to the valve driver 17 to fully close the electronic motor-driven valve 3 over t.sub.d as shown in FIG. 3 so as to prevent excessive cooling of the refrigeration space 9. This temperature control mode is called thermocycle mode.
With such refrigerant flow rate control, i.e. using a electronic motor-driven valve, refrigeration power of the evaporator 4 drops during the course of refrigeration due to the deposition of frost generated by the condensation of the water vapor in the damped air passed over the evaporator 4. Therefore, it is necessary to remove such frost, so that defrosting operations are periodically carried out. Instructions for such defrosting operations are provided from the controller 10 in the form of periodic defrosting signals (C) from means such as a timer. In response to a defrosting signal the electronic motor-driven valve 3 is fully closed to stop circulation of the refrigerant during defrosting. In FIG. 3 a defrosting signal C is output at time .tau..sub.1 at which defrosting is started. That is the electronic motor-driven valve 3 is fully closed to stop refrigeration, and a defrosting heater is turned on or a hot gas is supplied to the evaporator. Near the end of the defrosting cycle a sharp rise in temperature may be observed in the neighborhood of the evaporator 4, which is detected by some means such as a defrosting temperature sensor and the defrosting is terminated at .tau..sub.a.
At this point refrigeration is resumed, which lasts over the period T.sub.A. Such defrosting will repeat in such a way that the next defrosting starts at time .tau..sub.5 which is T after .tau..sub.1 and lasts period T.sub.B. The temperature in the refrigeration space rises during the defrosting.
A disadvantage encountered in this periodic defrosting is as follows. Low-temperature show cases in e.g. a supermarket are usually covered with so-called night caps over the refrigeration space storing goods when the shop is closed. Then the show case is insulated from the ambient air. Since the amount of frost deposited on an evaporator increases with the running period of the use of the show case and the frequency of the infiltration of ambient air into the show case, the amount of frost during such closed hours is extremely little. Hence, under such circumstances defrosting is not necessarily needed. Nevertheless the show case is forced to undergo defrosting operations, not only wastefully consuming electricity or hot gas, but also resulting in undesirable temperature rise in the refrigeration system and creating a disadvantageous influence on the quality of goods stored therein. Also, there may be some shopping hours when few customers open the show case and periodic defrosting is not needed. Periodic defrosting in such cases merely results in undesirable effects on the goods.